Essential trace elements, excretion

Copper is an essential trace element. It is required in the diet because it is the metal cofactor for a variety of enzymes (see Table 50—5). Copper accepts and donates electrons and is involved in reactions involving dismu-tation, hydroxylation, and oxygenation. However, excess copper can cause problems because it can oxidize proteins and hpids, bind to nucleic acids, and enhance the production of free radicals. It is thus important to have mechanisms that will maintain the amount of copper in the body within normal hmits. The body of the normal adult contains about 100 mg of copper, located mostly in bone, liver, kidney, and muscle. The daily intake of copper is about 2—A mg, with about 50% being absorbed in the stomach and upper small intestine and the remainder excreted in the feces. Copper is carried to the liver bound to albumin, taken up by liver cells, and part of it is excreted in the bile. Copper also leaves the liver attached to ceruloplasmin, which is synthesized in that organ. 

Selenium is an essential trace element involved in the detoxification of free radicals. Soluble selenium compounds are readily absorbed through the lungs and from the gut. They are methylated in the liver and rapidly excreted in the urine. Following very high exposure levels, methylated compounds may dso be eliminated from the body via the lungs, giving rise to a characteristic garlic smell on the hreath. 

Chelating agents, like all chemical compounds, exhibit toxic effects. These can arise from the fact that they increase the excretion of essential trace elements or from more subtle interactions. Thus all compounds which contain sulfhydryl compounds are capable of causing allergic reactions, such as the skin rash reported for DMSA (Grandjean et al. 1991), or the numerous problems which may arise from the continued administration of d-penicillamine such as nephrotic syndrome and anuria (Dubois et al. 1990). The administration of EDTA by itself may result in tetany due to the rapid drop in serum calcium which results, this being the reason for its customary administration as the calcium complex. 

Copper is an essential element to most life forms. In humans it is the third most abundant trace element only iron and zinc are present in higher quantity. Utilization of copper usually involves a protein active site which catalyzes a critical oxidation reaction, e.g., cytochrome oxidase, amine oxidases, superoxide dismutase, ferroxidases, dopamine-/ -hydrox-ylase, and tyrosinase. Accordingly, animals exhibit unique homeostatic mechanisms for the absorption, distribution, utilization, and excretion of copper (J). Moreover, at least two potentially lethal inherited diseases of copper metabolism are known Wilson s Disease and Menkes s Kinky Hair Syndrome (I). 

Trace metal disturbances may be due to the uremia per se. Indeed, as the urinary excretion route is an important pathway of elimination of many trace elements, i.e. silicon, strontium, aluminum,... impairment of the kidney will be an important determinant of their accumulation, whilst in the presence of a reabsorptive defect a number of trace elements, especially those that are reabsorbed because of their essential role, be lost resulting in a deficient state. The presence of proteinuria may reasonably result in losses of protein bound elements. It has also been shown also that residual renal funchon may importantly alter the accumulation and hence toxic effects of aluminum [2]. In uremia translocation of a particular metal from one tissue to another may also occur. As an example, under normal circumstances the kidney is an important target organ for cadmium. In chronic renal failure however, possibly as a consequence of a reduction in binding proteins (e.g. metallothionein), the concentrahon of cadmium in this tissue decreases to extremely low levels which... 

In biology and medicine, identification and characterization of species of essential (e.g., Fe, Cu, Zn, Se), toxic (e.g., Hg, Pb, Cd, As), and therapeutic elements (e.g., Pt, Au) in living organisms is arousing great interest these days. Also, in occupational medicine, speciation provides information about the volatile species (e.g., Hg) and inhalable particles at the workplace (e.g., Cr(VI) in dust particles), providing information about trace element toxicants absorption, distribution, reactivity, toxicity, and the final excretion after an occupational exposure to Pb, Cr, As, etc. 

Some of them, such as arsenic, lead, cadmium and mercury are frequently classified as toxic elements because their toxicity to man and animals is relatively high and their biological activity is largely confined to toxic reactions. However, all the trace elements can be toxic if consumed in sufficiently large quantities or for protracted periods of time, as we will learn this afternoon from Dr. Mertz, who will dis-curse on "essentiality and toxicity of heavy metals". I am sure that he will be able to supply us with further evidence that the toxicity or beneficial effect of a particular element can be greatly influenced by the extent to which other elements or compounds present in the diet affect its absorption, excretion, or metabolism. 

Chmielnicka J, Nasiadek M. 1991. Tissue distribution and urinary excretion of essential elements in rats orally exposed to aluminum chloride. Biol Trace Elem Res 31 131-138. 

Bioavailability" is a quantitative measure of the utilization of an element under specific conditions, and includes mechanisms such as absorption, transport to a site of metabolic/toxic activity, biotransformation to a metabolically active/toxic form, retention/accumulation, and excretion (McKenzie in Nriagu, 1984). Bioavailable species of a pollutants can be increased by small particle size or volatility, aque-ous/lipid solubility, complexation, and - for metals - by elements or complexes which mimic essential nutrients and thus are handled by specific active transport processes. Assessment of pollutant bioavailability in sediments is difficult due the compexicity of the system, and the following questions have been raised with respect to toxic trace metals (Luoma, 1983 Salomons Forstner, 1984 Campbell et al., 1988) ... 

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